Gas laser with mode control

ABSTRACT

A gas laser ( 1 ) has an optics system and a mode-masking diaphragm in the beam guiding chamber of the laser resonator. The optics system is adjustable between two settings in each of which any higher-order modes of the laser beam ( 3 ) are masked out. The Gaussian mode ( 9 ) can be generated in one of the two settings, and the ring mode ( 10 ) in the other setting. An appropriate control permits switching between Gaussian and ring modes depending upon the intended processing operation.

BACKGROUND OF THE INVENTION

This invention relates to a gas laser with an optics system and with amode-masking diaphragm in the beam guiding chamber of the laserresonator.

Lasers of this type have been previously known and are described, forexample, in EP 0 492 340.

A gas laser usually generates in its laser resonator a characteristicoscillation state, called the mode, which is essentially determined bythe length of the laser resonator, the diameter of the laser tube(s) andthe configuration of the electrodes. The design of a laser, and thus thetype of mode it generates, depends on the intended application. Formaterials processing there are two modes of particular significance—theTEM₀₀ mode (the so-called “Gaussian mode”) and the TEM_(01*) mode (theso-called “ring mode”).

The Gaussian mode permits focussing down to the smallest possible spotdiameter, a desirable feature for cutting thin sheet metal. The ringmode is more difficult to focus than the Gaussian mode, and generallyresults in a larger diameter for the focal spot. This is desirable forinstance when cutting thicker sheet metal since the cutting width islarge enough to permit the dross to be blown out. At the center of thering mode beam, there is a power minimum, reducing the thermal load inthe center of the optical elements, a feature which is important,particularly in the case of high-power systems.

Numerous attempts have been made in the past to set the mode in thelaser resonator of a gas laser in a defined manner. The design describedthe above-mentioned EP 0 492 340 employs as its mode-masking diaphragm,two longitudinally adjustable aperture disks in the laser resonator bywhich the diameter of the laser beam can be reduced. In an initialsetting, the two mode selector disks are positioned outside the laserbeam, allowing the full diameter of the beam to exit unobstructed to theoutside mirror. In a second setting, both mode diaphragm disks protrudeinto the beam path, reducing the diameter of the laser beam by about onehalf. A mechanically complex cylinder drive serves to move the modediaphragm disks in the longitudinal direction.

In contrast thereto, it is the object of this invention to provide anovel gas laser with a relatively simple structure to enable simpleswitching between two different modes for different applications.

SUMMARY OF THE INVENTION

It has now been found that the foregoing and related objects may bereadily attained in a gas laser having an optics system and amode-masking diaphragm in the beam guiding chamber of the laserresonator. The optics system includes at least two adaptive opticalelements adjustable between two settings, and the mode-masking diaphragmhas an aperture which is disposed between the adaptive optical elementswhich are selectable between either of two settings, in each of whichthe mode-masking diaphragm masks out any higher-order modes from thelaser beam.

Desirably, in at least one of the two settings of the adjustable opticalelements of the optics system, one adaptive optical element serves toexpand the laser beam while another adaptive optical element performsthe subsequent focussing of the laser beam. Preferably, in at least oneof the two settings of the optics system, two adaptive optical elementsserve to expand the laser beam while a third adaptive optical element(32) focuses the laser beam.

The adaptive optical elements are selected from the group comprising theoutput mirror of the laser resonator, the retro-mirror of the laserresonator, and one or more interpositioned beam deflectors.

The function of the mode-masking aperture is provided by the innerdiameter of a circular cross section of the beam guiding chamber. In oneembodiment, the circular section of the beam guiding chamber is providedby one or more laser tubes of the laser resonator. In anotherembodiment, the circular section of the beam guiding chamber is providedby a connecting block linking two adjacent laser tubes. In one of thesettings of the mode-masking diaphragm, the laser resonator isconfigured for generating a Gaussian mode and in the other setting theresonator is configured for generating a ring mode.

The laser also has a control device for setting the mode-maskingdiaphragm and the optics system, and at least two sets of parameters arestored in the control device for the two settings of the mode-maskingdiaphragm and optics system.

The advantage of the present invention lies in the fact that it makes itpossible to generate in the laser resonator the Gaussian mode in onesetting and the ring mode in the other setting, by a suitable controldevice which permits switching between the Gaussian and the ring mode asrequired for the intended application.

The switchable optics system preferably incorporates at least oneadaptive optical element such as an adaptive mirror whose reflectivesurface can change shape for instance when the pressure of the coolingwater is varied. This allows for the appropriate selection of therespectively desired curvature of the adaptive mirror by means of acontrol device.

The term “adaptive optical element” as employed herein refers to mirrorsand other optical elements of the laser assembly which can be modifiedin configuration or optical properties through the controlledapplication of external forces such as those which would change thetemperature water pressure, piezoelectric or mechanical forces.Illustrative of such devices is the temperature cooling water pressurecontrolled mirror illustrated and described in Giesen et al U.S. Pat.No. 5,020,895 granted Jun. 4, 1991.

In at least one of the two optical element settings, one or two adaptiveoptical elements, preferably convex, serve as the laser beam expanderwhile another adaptive optical element, preferably concave, serves forthe subsequent focussing of the laser beam.

For the adaptive optical elements it is possible to use elements thatare already parts of the optical path in the laser resonator, forinstance the output mirror and/or the retro-mirror of the laserresonator and/or one or several beam deflectors. If the retro-mirror isto be used for beam expansion or focussing, the fact must be taken intoaccount that, usually, it already has a curvature to assure a stablelaser resonator. The final curvature of the retro-mirror is then definedby simply adding the two curvatures together, with due considerationbeing given to the respective sign. Actively operable optical elementsinclude, for instance, a minimum of two neighboring adaptive beamdeflectors, possibly in conjunction with the retro-mirror. The preferredconfiguration of actively operable optical elements consists of threeadaptive beam deflectors.

Experiments with a double-squared convolution resonator have revealedthat with at least two optical elements particularly good results areobtained, meaning a laser beam in the Gaussian mode with a particularlyhigh beam quality, when one of the optical elements, especially whenconvex, is designed to expand the laser beam while the other opticalelement, especially when concave, serves for the subsequent focussing ofthe laser beam. Experiments with a double-squared convolution resonatorhave also revealed that with at least three optical elementsparticularly good results are obtained, i.e. a laser beam in theGaussian mode with a particularly high beam quality, when two of theoptical elements, especially when convex, serve to expand the laser beamwhile the other optical element, especially when concave, serves tofocus the laser beam. The radius of the one concave optical element ispreferably smaller than each of the radii of the convex optical elementswhile the radii of the convex optical elements are essentially matched.Particularly good results, i.e., a laser beam in the Gaussian mode withan especially high beam quality, can be obtained when one convex beamdeflector is located near the output mirror or when the output mirror isitself convex and the radii of the convex or concave mirrors are in therange from 10 m to 60 m.

In particularly preferred design versions of the invention, themode-masking aperture consists in the inside diameter of a round sectionof the beam guiding chamber. The advantage of this concept is that, incontrast to an aperture disk, there is no excited laser gas outside theexcitation area defined by the round section of the beam guidingchamber, i.e. outside the mode masking aperture, which might otherwisegive rise to undesirable amplification. Consequently, a crisp Gaussianmode can be generated in the laser resonator. Given the adjustableoptics, the laser beam can be expanded within the beam guiding chamberto a beam diameter of a magnitude where the inside diameter of the roundsection of the beam guiding chamber acts as the mode diaphragm formasking out higher-order modes.

In particularly preferred design versions of the invention, the laserresonator is so configured that in one setting of the mode diaphragm andthe optics it generates a Gaussian mode while in the other setting itgenerates a ring mode. This concept permits the selective setting of thelaser mode with the aid of beam-expanding and focussing optical elementsin the laser resonator, in the process of which the laser beam is shapedby these optical elements in such fashion that the round section of thebeam guiding chamber of the laser resonator itself functions as themode-masking aperture when the laser beam is expanded. Depending on theintended processing operation, a suitable control device can switchbetween the Gaussian and the ring mode. To that end, at least two setsof parameters for the two settings of the mode diaphragm and the opticsmay be stored in the control device.

Other advantages offered by this invention are evident from thedescription and the drawing. According to the invention, the featuresreferred to above and those explained further below may be appliedindividually or in any desired combination. The design versionsdescribed and illustrated are not to be viewed as a finite and finalenumeration but rather as examples serving to explain this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a diagrammatically illustrates a first embodiment of a gas laserof the present invention, with two adaptive optical elements forgenerating a Gaussian mode;

FIG. 1b shows the adaptive optic elements of FIG. 1a in a non-operativeposition for generating a ring mode;

FIG. 2a diagrammatically illustrates a second embodiment of the gaslaser of the present invention with three adaptive optical elements forgenerating a Gaussian mode;

FIG. 2b shows the adaptive optical elements of FIG. 2a in an alternateposition for generating a ring mode;

FIG. 3a illustrates a third embodiment of the gas laser of the presentinvention with three adaptive optical elements for generating a Gaussianmode; and

FIG. 3b shows the adaptive optical elements of FIG. 3a in an alternateposition for generating a ring mode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The laser resonator of the gas laser 1 illustrated in FIG. 1 includesthree laser tubes 2 which serve as the enclosure for a lasing gas suchas CO₂ or CO. The laser tubes have a round cross section and may consistof quartz or of a ceramic material. Any two juxtaposed laser tubesextend at a right angle relative to each other, for a combined U-shapedconfiguration. The laser beam 3 generated in the laser resonator isreflected between a planar (flat) retro-mirror 4 and a planar (flat)output mirror 5 and is redirected at 90° angle between any two lasertubes 2 by the beam deflectors 6, 7.

The optical path of the laser beam 3 is indicated in greatly simplifiedfashion. The two beam deflectors 6, 7 are adaptive mirrors with avariable radius of curvature which can be selected by means of a controldevice 8.

From the output mirror 5 in the embodiment of FIG. 1a, the laser beam 3is expanded by the beam deflector 6 to provide a in convex surface to apoint where, with the exception of the Gaussian mode 9, all other modesof the laser beam 3 including especially the ring mode 10 are masked outof the laser beam 3 by the central laser tube 2. The beam diameter 11 ofthe laser beam 3 in the central laser tube 2 is thus trimmed to theinner diameter 12 of the central laser tube 2, i.e., the central lasertube 2 functions as the mode aperture. There is no excited laser gasoutside the mode area, as a result of which a sharply defined Gaussianmode 9 is generated in the laser resonator.

The beam deflector 7 with a concave surface, refocuses the expandedlaser beam 3 to a point where the laser tube 2 located between the beamdeflector 7 and the retro-mirror 4 will not act as a mode aperture forthe Gaussian mode. The same applies to the laser tube 2 located betweenthe output mirror 5 and the beam deflector 6. In this embodiment, twoneighboring laser tubes 2 are linked by a connecting block 13 which ispart of the laser resonator.

In FIG. 1b, the adaptive beam deflectors 6, 7 are set to function asplane mirrors so that the laser beam 3 is not expanded within thecentral laser tube 2. The architecture of the laser resonator is suchthat, in this case, it generates the ring mode 10. Simple “switching” ofthe contour of the adaptive mirrors 6, 7 allows the selective generationof the Gaussian mode 9 or ring mode 10 in the laser resonator.

For switching between the Gaussian mode and the ring mode, the gas laser20 depicted in FIG. 2 employs two adaptive beam deflectors 21, 22 andone adaptive retro-mirror 23, and their settings are respectivelydetermined by the control device 24. In the configuration of FIG. 2awhich has convex and concave beam deflectors 21, 22 and a convexretro-mirror 23, the laser beam 3 is expanded in the central laser tube2 for mode-limiting and, accordingly, only the Gaussian mode 25 isgenerated by the laser resonator. In the reconfiguration of FIG. 2b, thebeam deflectors 21, 22 and the retro-mirror 23 are set to act as planar(flat) mirrors so that no expansion of the laser beam 3 takes placewithin the central laser tube 2. Thus, the architecture of the laserresonator is such that in this case it generates the ring mode 26.Simple “switching” of the adaptive mirrors 21, 22, 23 allows theselective generation of the Gaussian mode 25 or the ring mode 26 in thelaser resonator.

For switching between the Gaussian mode and the ring mode, the gas laser30 illustrated in FIG. 3 employs three adaptive beam deflectors 31, 32,33, and their respective contours are determined by the control device34. In the configuration of FIG. 3a, using the combination convex beamdeflectors 31, 33 and a concave beam deflector 32, the laser beam 3 isexpanded in the laser tube 2 for mode-limiting and correspondingly onlythe Gaussian mode 35 is generated in the laser resonator. The radii ofcurvature of the beam deflectors 31, 32, 33 are so chosen that theycompensate for astigmatic errors. In the configuration of FIG. 3b, allbeam deflectors 31, 32, 33 are set to act as plane mirrors so that noexpansion of the laser beam 3 takes place within the laser tube 2. Thearchitecture of this laser resonator generates a ring mode 36.

Thus, it can be seen that simple “switching” of the adaptive mirrors 31,32, 33 allows the selective generation of the laser beam in the Gaussianmode 35 or the ring mode 36 in the laser resonator.

Having thus described the invention, what is claimed is:
 1. A gas laser(1,20,30) having an optics system and a mode-masking diaphragm in thebeam guiding chamber of the laser resonator, said optics systemincluding at least a pair of adaptive optical elements disposed atopposite sides of said mode masking diaphragm and adjustable between:two settings, said mode-masking diaphragm having an aperture whichmasks out any higher-order modes from the laser beam (3)passingtherethrough in either of said settings, in at least one of the twosettings of said adjustable optical elements of said optics system, oneadaptive optical element (6, 21, 31) serves to expand the laser beam (3)while another adaptive optical element (7, 22, 32) performs thesubsequent focussing of the laser beam (3).
 2. The gas laser inaccordance with claim 1, wherein said optics system includes a thirdadaptive optical element, and in which in at least one of the twosettings of the optics system, two adaptive optical elements (31, 33)serve to expand the laser beam (3) while a third adaptive opticalelement (32) focuses the laser beam (3).
 3. The gas laser in accordancewith claim 1, in which the adaptive optical elements are selected fromthe group comprising the output mirror (5) of the laser resonator, theretro-mirror (4,23) of the laser resonator and one: or moreinterpositioned beam deflectors (6, 7, 21, 22, 31, 32, 33).
 4. The gaslaser in accordance with claim 1, in which the function of themode-masking aperture is provided by the inner diameter (12) of acircular cross section of the beam guiding chamber.
 5. The gas laser inaccordance with claim 4, wherein the circular section of the beamguiding chamber is provided by one or more laser tubes (2) of the laserresonator.
 6. The gas laser in accordance with claim 4, wherein thecircular section of the beam guiding chamber is provided by a connectingblock (13) linking two adjacent laser tubes (2).
 7. The gas laser inaccordance with claim 1 in which in one of the settings of the adaptiveoptical elements of said optics system, the laser resonator isconfigured for generating a Gaussian mode (9,25, 35) and in the othersetting the resonator is configured for generating a ring mode(10,26,36).
 8. The gas laser in accordance with claim 1 in which, in oneof the settings of said adaptive optical elements of said optics systemthe laser resonator is configured for generating a Gaussian mode (9, 25,35) and in the other setting the resonator is configured for generatinga ring mode (10,26,36).
 9. The gas laser in accordance with claim 1including a control device (8, 24, 34) for setting-said adaptive opticalelements of said optics system and in which at least two sets ofparameters are stored in said control device (8, 24, 34) for the twosettings of said optics system.
 10. The gas laser in accordance withclaim 9 including a control device (8, 24, 34) for setting themode-masking diaphragm and the optics system and in which purpose atleast two sets of parameters are stored in said control device (8, 24,34) for the two settings of the mode-masking diaphragm and opticssystem.
 11. The gas laser in accordance with claim 1 wherein saidadaptive optical elements are set as plane mirrors to generate a laserbeam in the ring mode.